Use the Settings option in the Alexa app on your iPhone or iPad to enable Brief Mode for Alexa repsonses

Unlike Follow-Up Mode, Brief Mode applies to all of your Alexa-compatible devices. Once enabled, Alexa will be a little more terse and beep more everywhere. If you decide you’d rather hear Alexa say “OK” after turning on your lights you can jump back into the settings in the Alexa app and turn Brief Mode off again.